Information about the factors that regulate the formation of synaptic connections is fundamental to an understanding of how neural circuits are established and maintained. A particularly accessible period in which to study this regulation is during metamorphosis, when there is a major reorganization of many parts of the nervous system. This reorganization could be accomplished by the replacement of one set of neurons by another, or by the retention of neurons whose functions are respecified. In order to examine the relative importance of these two mechanisms in amphibian metamorphosis, this project will focus on a pair of neurons, the Mauthner (M) cells, in the African clawed frog, Xenopus laevis. The M cells are uniquely identifiable in the brain of every individual and are present during embryonic, larval, and adult stages. In embryos and larvae, the M cells synapse on spinal motoneurons of the body axis to produce a rapid tailflip that is the first part of an escape behavior. The function of these cells in the tailless adult is unknown. This proposal will test the hypothesis that the M cells are functionally respecified during metamorphosis, by examining whether they establish synapses with limb motoneurons that develop during this period. Two approaches will be used to address this question. First, the overlap between M axon collaterals and the dendritic arborization of axial and limb motoneurons will be assessed throughout the metamorphic period. Horseradish peroxidase (HRP) will be injected intracellularly into the M axons in tadpoles whose axial or limb motoneurons have been labeled retrogradely with HRP by prior exposure to the periphery. Spinal cords will be sectioned and processed histochemically, and the labeled neurons will be traced and reconstructed. Second, intracellular recordings will be made from axial and limb motoneurons while one of the M axons is stimulated with a second intracellular electrode. A battery of tests will be used to determine whether any potentials elicited in the motoneurons are transmitted monosynaptically from the M cell. HRP will be injected into motoneurons that are postsynaptic to the M cell, to reveal the extent of their overlap with the M axon. if these experiments indicate that the M cells are respecified to synapse with limb motoneurons, similar tests will be conducted to determine the development stage during which these synapse are formed and to ascertain whether they are maintained in the adult. A long- term goal of the research will be to identify the stimulus for this respecification.